1. Field of the Invention
The present invention relates to a grounding method for inverter units for driving motors used as drive sources for machines, such as machine tools, various industrial machines, robots, etc., and to an inverter unit, and a machine using the inverter unit.
2. Description of the Related Art
An inverter unit is used as a drive control circuit for driving a motor that is used as a drive source for machines, such as machine tools, various industrial machines, robots, etc. The inverter unit urges a switching element to turn on or off current to be supplied to each winding of the motor, thereby drivingly controlling the motor. Thereupon, the switching current generates noise that consists mainly of high frequency. This noise influences other control circuits.
In order to restrain the influence of the noise that is generated by the switching current, a shielded cable is used to connect the inverter unit and each winding of the motor. In known configurations (e.g., Japanese Patent Application Laid-Open Nos. 2002-281765 and 2001-286152), a shielded cable is used as a connecting wire that connects an inverter unit and each winding of a motor, one end of the cable being grounded, for example.
The inverter unit is provided with a control circuit for controlling it and control circuits for various sensors for detecting the conditions of the motor that is drivingly controlled by means of the inverter unit. The sensors include ones that are attached to the motor and detect the position and speed of the motor, a sensor for detecting the pole position of a rotor, a sensor for detecting the temperature of the motor, etc.
In some cases, control circuits that include the sensor circuits in the inverter unit and connecting wires that connect the control circuits and the sensors may operate wrongly under the influence of the noise that is generated by the on-off operation of the switching element of the inverter unit. Conventionally, as shown in FIG. 2, therefore, a sensor circuit 2 in an inverter unit 1 and a sensor 3 are connected by means of a shielded cable 4, and a shield braid 5 of the shielded cable 4 is connected to an earth plate 7 that is located outside the inverter unit 1. Thus, the influence of noise upon the signal wire that connects the sensor circuit 2 and the sensor 3 is restrained.
On the other hand, a 0V of a DC circuit system that includes the sensor circuit 2 and the like in the inverter unit 1 is separately connected to an earth plate 8 lest the circuit system 2 be influenced by the noise.
According to an alternative method shown in FIG. 3, a shield braid 5 of a shielded cable 4 that connects a sensor circuit 2 in an inverter unit 1 and a sensor 3 are connected to an earth plate 7. A 0V of a circuit system that includes the sensor circuit 2 and the like in the inverter unit 1 is trained in the inverter unit and connected to an earth wire of the inverter unit.